When a motor is operated by using an inverter, a noise is generated by the motor due to a switching operation of a transistor mounted on a circuit. In order to reduce the noise generated by the motor, a carrier frequency of the inverter is conventionally increased. While a shaft voltage is generated in the shaft of the motor by electromagnetic induction, the shaft voltage increases with increase of the carrier frequency of the inverter, and accordingly, an electrical potential difference between the inner ring and the outer ring of the bearing, which supports the shaft, increases. As a result, a substantial amount of electrical current flows in the bearing. This causes corrosion on an orbit surface which extends in a circumferential direction of the inner ring and the outer ring of the bearing and a rolling surface of a rolling element interposed between the inner ring and the outer ring, which is called electric corrosion. When the electric corrosion is generated on the bearing, a noise is generated by the bearing or a durability of the bearing is reduced. In order to prevent the electric corrosion of the bearing, techniques have been proposed as described below.
Patent Literature 1 discloses a brushless motor in which a stator core and a bracket that supports a bearing is short-circuited via a core connecting terminal (cable) and a bracket connecting terminal (cable). This conventional technique attempts to decrease a voltage applied to the bearing by a short circuit between the stator core and the bracket and reduce electric corrosion of the bearing.
Patent Literature 2 discloses a fan driving electric motor in which an insulator is interposed between a metal motor case and a bearing which is disposed in the motor case. An electric current which flows in the shaft is transmitted to an inner ring of the bearing supporting the shaft, and then, flows from the inner ring via a rolling element to be transmitted to the outer ring and then to the motor case. The technique of Patent Literature 2 attempts to prevent an electric current from flowing from the outer ring to the motor case by providing an insulator between the outer ring of the bearing and the motor case and reduce electric corrosion of the bearing.